Zinc oxide voltage-dependent resistors are a new technology which has been developed since the 1970s. The voltage-dependent resistor has a certain switching voltage (called voltage-dependent voltage). Below this voltage, the voltage-dependent resistor will have a very high resistance, which is equivalent to the insulation state. When given a high voltage impulse (higher than the voltage-dependent voltage), the voltage-dependent resistor will have a very low resistance, which is equivalent to the short circuit state. When the voltage which is higher than the voltage-dependent voltage disappears, it returns to its high resistance state.
If the zinc oxide voltage-dependent resistor is installed in electrical equipment, when an excessively high voltage, which is higher than its rated work voltage, is applied to the voltage-dependent resistor, the voltage-dependent resistor may be broken down by the excessively high voltage, resulting in an over-high current flowing through the voltage-dependent resistor, thus causing the voltage-dependent resistor to explode and burn, which becomes a safety concern for the surrounding electrical equipment.
Chinese patent No. 02222055.0 discloses a voltage-dependent resistor with thermal protection, which includes a housing and a voltage-dependent resistor enclosed by the housing. The voltage-dependent resistor is composed of a voltage-dependent ceramic substrate and an insulated enveloping layer which encloses the voltage-dependent ceramic substrate. The voltage-dependent resistor has a first lead-out electrode and a second lead-out electrode, wherein one end of the first lead-out electrode and one end of the second lead-out electrode are respectively connected to the two electrode leads of the voltage-dependent ceramic substrate, and the other end of the first lead-out electrode and the other end of the second lead-out electrode extends beyond the housing, and further has a metal spring and a third electrode, wherein the metal spring is arranged in the housing, the voltage-dependent resistor has a metal heat conductor connecting to an inner electrode of the voltage-dependent ceramic substrate, one end of the metal spring is welded to the metal heat conductor via a low melting-point metal, and the other end of the metal spring is secured onto the housing and connected to the third electrode, and the third electrode has a leading end extending beyond the housing.
Chinese patent No. 200620155019.4 discloses a voltage-dependent resistor with overheat protection structure which is mainly composed of a housing, pins arranged in the housing, and spring tabs connecting to the pins. One of the pins at the end of the voltage-dependent resistor which is serially connected to the overheat protection structure is welded to the free end of the spring tab via a metal with a low melting point. When such voltage-dependent resistor with overheat protection structure is overheated, the heat energy of the voltage-dependent resistor can disconnect the protection device in time, so as to withstand the impulse of a strong lightning current.
Chinese patent No. 200610168133.5 also discloses a voltage-dependent resistor with overheat protection function, wherein an insulated bracket is installed on the body of the voltage-dependent resistor component, and the heating fuse and the pin are electrically connected with each other and are configured on the insulated bracket; when the body experiences an abnormal rise in temperature when receiving a variety of over-high voltages, the heat is conducted to the fuse rapidly thanks to the increased heat-conducting area of the insulation bracket, and at the same time the temperature of the heating fuse rises due to the overheat so that when the temperature is higher than its welding point, causing it to become a fusion liquid state, which, combined with the capillary action resulting from several grooves set in the insulated bracket, further causes the fusion liquid-phase fuse to spread, melt and separate rapidly, thereby protecting the body from igniting and burning, enabling timely circuit breaking to protect the electronic components in the circuit from being damages.